The information recording media of the prior art include a phase change (or phase transition) type information recording medium that utilizes a phenomenon of phase change taking place in a recording layer (phase change material layer). Among the phase change type information recording media, there is the information recording medium that optically records, erases, rewrites and reproduces information by using a laser beam (which may hereinafter be referred to simply as an optical information recording medium). Information is recorded on the optical information recording medium by irradiating a phase change material of a recording layer with the laser beam so that heat generated thereby causes, for example, change of state between crystal phase and amorphous phase. The recorded information is read by detecting the difference in reflectivity between the crystal phase and the amorphous phase.
The optical information recording media also include a rewritable information recording medium from and on which information can be erased and can be rewritten. In this medium, the initial state of the recording layer is crystal phase in general. To record information on this medium, it is irradiated with a laser beam of high power (recording power) so as to melt the recording layer and then cool it down quickly, thereby turning the portion of the recording layer that has been irradiated with the laser into amorphous phase. In contrast, to erase information from this medium, it is irradiated with a laser beam having a power (erasure power) lower than that of recording, so as to raise the temperature of the recording layer and then gradually cool it, so as to turn the portion of the recording layer that has been irradiated with the laser into crystal phase. Accordingly, it is made possible to record new information or rewrite information while erasing the information already recorded, by irradiating the recording layer of the rewritable information recording medium with a laser beam that is power-modulated between a high power level and a low power level. Turning the recording layer into crystal phase requires it to maintain the recording layer at a temperature at which it changes to crystal phase (crystallization temperature) for a certain period of time (crystallization time). A shorter crystallization time makes it possible to delete and rewrite information in a shorter period of time, namely enables high-speed erasure and rewriting.
The phase change type information recording media also include a write-once information recording medium on which information is recorded only once, and from and on which information cannot be erased or rewritten, wherein the initial state of the recording layer is amorphous phase in general. To record information on the write-once information recording medium, it is irradiated with a laser beam of high power (recording power) so as to raise the temperature of the recording layer and is then cooled down gradually, thereby turning the portion that has been irradiated with the laser into crystal phase.
There is also a type of phase change type information recording medium whereon information is recorded by applying electrical energy (for example, electric current) instead of irradiation of laser beam, so as to cause state change of a phase change material of the recording layer by the Joule heat generated thereby. Information is recorded on this information recording medium by causing change of the state between crystal phase (low resistance) and amorphous phase (high resistance) in the phase change material of the recording layer by the Joule heat generated by flowing the electric current. The recorded information is read by detecting the difference in electrical resistance between crystal phase and amorphous phase.
As an example of the phase change type information recording medium, there is a 4.7 GB/DVD-RAM. The 4.7 GB/DVD-RAM, as depicted as an information recording medium 12 in FIG. 15, has 7-layer constitution including a first dielectric material layer 2, a first interface layer 3, recording layer 4, a second interface layer 5, a second dielectric material layer 6, a light absorption compensating layer 7 and a reflective layer 8 which are provided in this order on a substrate 1 when viewed from the side where laser beam enters.
The recording layer 4 is formed from a fast-crystallizing material that contains (Ge—Sn)Te—Sb2Te3, which is prepared by substituting a part of Ge with Sn in a GeTe—Sb2Te3 quasi-binary phase change material, that is a mixture of compounds GeTe and Sb2Te3 (refer to, for example, Patent Document 1). It is made possible to rewrite information at a higher speed by using a GeTe—Bi2Te3 quasi-binary phase change material, that is a mixture of compounds GeTe and Bi2Te3 (refer to, for example, Patent Document 2). By using these materials, it is made possible to achieve not only high initial rewriting performance but also excellent archival characteristics (capability to reproduce recorded information after a long period of storage) and excellent overwrite archival characteristics (capability to erase or rewrite recorded information after a long period of storage).
The first dielectric material layer 2 and the second dielectric material layer 6 have optical functions to increase the efficiency of the recording layer 4 to absorb light by regulating the optical distance and increase the difference in the reflectivity between crystal phase and amorphous phase so as to increase the intensity of signals. These dielectric material layers 2 and 6 also have a thermal function to thermally insulate the substrate 1, a dummy substrate 10, etc. that are vulnerable to heat, from the recording layer 4 that is heated to a high temperature during recording. (ZnS)80(SiO2)20 (mol %) that has been used in the prior art is an excellent dielectric material having transparency, high refractive index, low heat conductivity, high thermal insulation, good mechanical characteristics and high humidity resistance.
The reflective layer 8 has an optical function to increase the amount of light absorbed by the recording layer 4. The reflective layer 8 also has a thermal function to quickly dissipate the heat generated in the recording layer 4 and facilitate the phase change of the recording layer 4 into amorphous phase. The reflective layer 8 further has a function to protect the multi-layer film from the operating environment.
The first interface layer 3 and the second interface layer 5 have the function to prevent material transfer from occurring between the first dielectric material layer 2 and the recording layer 4 and between the second dielectric material layer 6 and the recording layer 4. The material transfer is the diffusion of S (sulfur) into the recording layer in the course of repetitive irradiation of the recording layer 4 with the laser beam during recording and rewriting cycles, in the case where the first dielectric material layer 2 and the second dielectric material layer 6 are formed from (ZnS)80(SiO2)20 (the subscript represents the proportion of the respective component in mol %). Diffusion of S into the recording layer causes deterioration of overwrite cycle-ability. In order to prevent overwrite cycle-ability from deteriorating, it is preferable to use a nitride that contains Ge to form the first interface layer 3 and the second interface layer 5 (refer, for example, to Patent Document 3).
The 4.7 GB/DVD-RAM was successfully commercialized as high overwrite cycle-ability and high reliability were achieved by making use of the technologies described above,
In the meantime, various technologies have been introduced for the purpose of further increasing the recording capacity of the information recording medium. With regards to the optical information recording medium, for example, such a technology has been developed that employs a blue-violet laser that has a shorter wavelength than the red laser and an objective lens having a higher numerical aperture (NA) with a decreased thickness of the substrate on the side whereon the laser beam is incident, thereby to apply a laser beam having a smaller spot so as to record information with a higher density.
Such a technology has also been introduced that increases the recording capacity two-fold by using an optical information recording medium that has two information layers, and information is recorded on and reproduced on and from the two information layers by means of a laser beam that is incident on only one side of the medium (refer, for example, to Patent Document 4). In the two-layer information recording medium, information is recorded on and reproduced from the information layer located farther from the surface whereon the laser beam enters (hereafter referred to as the second information layer) by means of a laser beam that has transmitted through the information layer located nearer to the surface whereon the laser beam enters (hereafter referred to as the first information layer). Therefore, it is necessary to make the recording layer and the reflective layer of the first information layer extremely thin, so as to increase the light transmittance.    Patent Document 1: Japanese Patent Publication No. 2584741 (pp. 1-5, FIG. 1)    Patent Document 2: Japanese Patent Publication No. 2574325 (pp. 1-5, FIG. 1)    Patent Document 3: Japanese Unexamined Patent Publication (Kokai) No. H10-275360 (pp. 2-6, FIG. 2)    Patent Document 4: Japanese Unexamined Patent Publication (Kokai) No. 2000-36130 (pp. 2-11, FIG. 2)
However, there has been such a problem that making the recording layer (for example, GeTe—Bi2Te3) thinner results in lower capability of the recording layer to crystallize, leading to lower erasability. When the recording layer and the reflective layer are made extremely thin so as to increase the light transmittance, absorption of light by the recording layer decreases and higher energy (laser power) is required to record information, which means lower recording sensitivity.